I was looking at the Eizo mostly because of the rep of the company and five year warranty (and I also might've tiny bit taken in by all the "Near Zero input lag!" stuff they've got going on, figured it would make it an extremely responsive IPS).
A cheaper monitor will do just as good. I guarantee you will not notice the difference. The only difference that you will notice is the price lol.
PCPartPicker estimates the system wattage at 493- Would it really be okay to run it that close to spec for a 550W psu? I understand that overall power ratings for components are not likely to go up (I'll probably not end up doing double video cards, only upgrading to a new one), but aren't PSU's also significantly more energy efficient the lower the load is relative to their rating? Also, what exactly are quality psus, I can't make heads or tails of it.
550W is more than plenty. PCPartPicker overestimates the required wattage. If you want, you're more than welcome to bump it up to 650 or even 750 if you plan on doing crossfire in the future. PSUs typically work the best at a little more than half load. Quality PSUs depend on a couple of things....efficiency and OEM. Seasonic, XFX, Rosewill, and certain Corsair models are among the best brands. You also want to make sure your PSU uses a single, strong 12v rail. "Strong" depends on amperage. This info is usually listed on most vendors' websites.
I take it that MSI is on par with the toxic version by Sapphire? Are there any real differences besides price?
Pretty much the same. I'd go for MSI since it's cheaper and they are well known for their overclocking abilities. Sapphire's cards are usually voltage locked, not sure if this applies to this particular card though. I recommend searching for reviews on Google and comparing them. Look for the one with the lowest temps and acoustic levels, while being the best value for your money.
It seems you also cut out the SSD- Was that purely a budget choice? Anything wrong with that particular SSD over others? It does make sense to forgo the SSD for now but I will get one soon enough so it's relevant to me.
I cut out the SSD in favor of much better components. You can always add a SSD in your build later. However, adding a better motherboard, PSU, CPU, etc can be a pain and more expensive in the long run. The particular one you chose is actually a great SSD for the money. It's reliable and fast. I would recommend that you save up for a 250GB SSD. 120GB will fill up fast.
The monitor you mentioned is said to have light bleed problems- I haven't faced light bleed in ages so I don't know whether that should make me particularly worried. Could you ELI5 what I should be looking at in IPS monitors? Any good resources for making comparisons?
I only saw one customer review that mentioned this. If you're still not sure, then I'll list some more options for you to consider below. I just can't recommend you buy a 1080p monitor for $400, regardless of warranties and company rep. You could get a 1440p monitor at that price. However, I think 1080p better suits your budget and your overall build. Just look out for things such as the backlight bleed you mentioned. Research reviews on Google. Make sure it includes features that you require (resolution, screen size, lighting method, etc).
Monitors:
http://uk.pcpartpicker.com/part/asus-monitor-vn248h
http://uk.pcpartpicker.com/part/lg-monitor-27ea33v
http://uk.pcpartpicker.com/part/lg-monitor-ips231pbn
http://uk.pcpartpicker.com/part/viewsonic-monitor-vp2365led